A conventional ultrasonic probe comprises a transducer package which must be supported within a probe housing 2. As shown in FIG. 1, a conventional transducer package comprises an array 4 of narrow transducer elements. Each transducer element is made of piezoelectric ceramic material. The piezoelectric material is typically lead zirconate titanate (PZT), polyvinylidene difluoride, or PZT ceramic/polymer composite.
Typically, each transducer element has a metallic coating on opposing front and back faces to serve as electrodes. The metallic coating on the front face serves as the ground electrode. The ground electrodes of the transducer elements are all connected to a common ground. The metallic coating on the back face serves as the signal electrode. The signal electrodes of the transducer elements are connected to respective electrical conductors formed on a flexible printed circuit board (PCB) 6.
During operation, the signal and ground electrodes of the piezoelectric transducer elements are connected to an electrical source having an impedance Z.sub.s. When a voltage waveform is developed across the electrodes, the material of the piezoelectric element contracts at a frequency corresponding to that of the applied voltage, thereby emitting an ultrasonic wave into the media to which the piezoelectric element is coupled. Conversely, when an ultrasonic wave impinges on the material of the piezoelectric element, the latter produces a corresponding voltage across its terminals and the associated electrical load component.
In conventional applications, each transducer element produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter (not shown). The pulses are transmitted to the transducer elements via the flexible PCB 6. This ultrasonic energy is transmitted by the probe into the tissue of the object under study. The ultrasonic energy reflected back to transducer element array 4 from the object under study is converted to an electrical signal by each receiving transducer element and applied separately to a receiver (not shown).
The transducer package also comprises a mass of suitable acoustical damping material having high acoustic losses, e.g., silver epoxy, positioned at the back face of the transducer element array 4. This backing layer 8 is coupled to the rear face of the transducer elements to absorb ultrasonic waves that emerge from the back side of each element so that they will not be partially reflected and interfere with the ultrasonic waves propagating in the forward direction.
In the transmission mode, the piezoelectric ceramic of the transducer elements alternately contracts and expands in response to electrical signals received from a pulser circuit (not shown) via coaxial cables (not shown) electrically connected to the flexible PCB 6. The resulting compression waves propagate in both the forward and rearward directions, with the rearward-propagating compression waves being damped by the backing layer 8.
In the receiving mode, the piezoelectric ceramic of the transducer elements alternately compresses and expands in response to compression waves reflected back to the probe by the object being ultrasonically examined. These waves are transduced into electrical signals which are carried to a receiver circuit (not shown) by the flexible PCB 6 and the coaxial cable connected thereto.
The transducer element array 4, flexible PCB 6 and backing layer 8 are bonded together in a stack-up arrangement which is secured inside a four-sided array case 10, which protects the fragile transducer elements during probe assembly. The perimeter of the array case 10 may be surrounded with a sheet of electrically conductive foil to form an electrical shield 20. The transducer package or stack is mounted inside the probe housing 2 with internal spaces filled with potting material 18.
Typically, a first acoustic impedance matching layer 12 is laminated to the bottom surface of the transducer array, as shown in FIG. 1. Optionally, a second acoustic impedance matching layer 14 can be laminated to the first acoustic impedance matching layer 12. These impedance matching layers transform the high acoustic impedance of the transducer elements to the low acoustic impedance of the human body or water, thereby improving the coupling with the medium in which the emitted ultrasonic waves will propagate.
The front face of the outermost acoustic impedance matching layer is conventionally bonded to the planar rear face of a positive cylindrical lens 16 using an acoustically transparent thin layer of silicone adhesive. The front face of the lens has a convex cylindrical contour which focuses the ultrasound emitted from the outermost acoustic impedance matching layer. Two conventional lens geometries are depicted in FIGS. 2A and 2B.
Lens 16 is conventionally made of silicone rubber. This layer of silicone rubber serves three purposes: (1) acoustic focusing (due to its lens-shaped cross section and its low acoustic velocity material properties); (2) providing a chemical barrier to protect the transducer elements from attack by gels, body fluids, cleaning agents, etc.; and (3) providing an electrical barrier to protect the patient from the electrically active transducer elements.
Although a lens made of silicone rubber will adequately serve the above-described functions of an outer layer, the outer surface of the silicone rubber is easily damaged by abrasion, impact and some harsh chemicals commonly used in clinical environments. Such damage to the outer layer of the transducer package may result in a loss of acoustic focusing integrity and/or breach of the chemical/electrical barrier, thereby causing irreparable probe damage and/or patient safety concerns.